In many applications an electrical current has to be measured and monitored. For example, in vehicles having a battery used for various purposes, such as starting an engine or powering electrical systems such as lights and instruments, current is measured and monitored for instrumentation control purposes. For example, U.S. Pat. No. 4,937,523 entitled "Method for Monitoring Automotive Battery Status", which is assigned to the assignee of the subject application, describes an analysis system in which current is measured and monitored during operation of an automobile to determine capacity, state of charge and certain fault conditions of the vehicle's battery. Such a system can also be used for other types of vehicles, such as aircraft, buses, etc.
One technique for electrical current measurement and/or monitoring the current flow in the electrical systems of automotive, aircraft and other vehicles requires a sensor that effectively measures the magnetic field caused by a current flowing through a conductor. Some of the commonly used sensor types are Hall effect and inductive devices. Another technique, used in the aforesaid patent, is that of a precision resistance shunt of a known resistance value which is placed in series with a part of the conductor carrying the current to be measured. By measuring the voltage across the shunt and knowing its resistance, the current can be calculated.
While the use of a shunt is adequate for many purposes, it has disadvantages in that it requires the cost of the shunt itself and additional electrical connections. More importantly, the shunt results in a power loss which is a product of the square of the current value times the resistance value of the shunt. For example, if a one ohm shunt is used to measure current flow in the electrical system of an automobile, the power loss due to the resistance heating of the shunt would be substantial and could cause unnecessary fuel consumption in the vehicle.
Accordingly, it would be desirable to be able to measure current in a vehicle having a battery without the necessity of using an additional element such as a shunt or another type of sensor.